Hydraulic fracturing of petroleum recovery wells enhances the extraction of fluids from low permeable formations due to the high permeability of the induced fracture and the size and extent of the fracture. A single hydraulic fracture from a well bore results in increased yield of extracted fluids from the formation. The production of petroleum fluids, however, is typically from the region of the formation in close proximity to the fracture and thus large quantities of the petroleum fluids in the formation are not recovered. Creating multiple fractures at differing orientations or azimuths from a single well bore will further increase the yield from the well and result in a much greater recovery of the petroleum reserves from the formation.
Turning now to the prior art, hydraulic fracturing of subsurface earth formations to stimulate production of hydrocarbon fluids from subterranean formations has been carried out in many parts of the world for over fifty years. The earth is hydraulically fractured either through perforations in a cased well bore or in an isolated section of an open bore hole. The horizontal and vertical orientation of the hydraulic fracture is controlled by the compressive stress regime in the earth and the fabric of the formation. It is well known in the art of rock mechanics that a fracture will occur in a plane perpendicular to the direction of the minimum stress, see U.S. Pat. No. 4,271,696 to Wood. At significant depth, one of the horizontal stresses is generally at a minimum, resulting in a vertical fracture formed by the hydraulic fracturing process. It is also well known in the art that the azimuth of the vertical fracture is controlled by the orientation of the minimum horizontal stress in consolidated sediments and brittle rocks.
At shallow depths, the horizontal stresses could be less or greater than the vertical overburden stress. If the horizontal stresses are less than the vertical overburden stress, then vertical fractures will be produced; whereas if the horizontal stresses are greater than the vertical overburden stress, then a horizontal fracture will be formed by the hydraulic fracturing process.
Techniques to induce a preferred horizontal orientation of the fracture from a well bore are well known. These techniques include slotting, by either a gaseous or liquid jet under pressure, to form a horizontal notch in an open bore hole. Such techniques are commonly used in the petroleum and environmental industry. The slotting technique performs satisfactorily in producing a horizontal fracture, provided that the horizontal stresses are greater than the vertical overburden stress, or the earth formation has sufficient horizontal layering or fabric to ensure that the fracture continues propagating in the horizontal plane. Perforations in a horizontal plane to induce a horizontal fracture from a cased well bore have been disclosed, but such perforations do not preferentially induce horizontal fractures in formations of low horizontal stress. See U.S. Pat. No. 5,002,431 to Heymans.
Various means for creating vertical slots in a cased well bore have been disclosed. The prior art recognizes that a chain saw can be used for slotting the casing. See U.S. Pat. No. 1,789,993 to Switzer; U.S. Pat. No. 2,178,554 to Bowie, et al., U.S. Pat. No. 3,225,828 to Wisenbaker; and U.S. Pat. No. 4,119,151 to Smith. Installing pre-slotted or weakened casing has also been disclosed in the prior art as an alternative to perforating the casing, since such perforations can result in a reduced hydraulic connection of the formation to the well bore due to pore collapse of the formation surrounding the perforation. See U.S. Pat. No. 5,103,911 to Heijen. These methods in the prior art were not concerned with the azimuth orientation of two opposing slots for the preferential initiating of a vertical hydraulic fracture at a predetermined azimuth orientation. It has been generally accepted in the art that the fracture azimuth orientation cannot be controlled by such means. These methods were an alternative to perforating the casing to achieve better connection between the well bore and the surrounding formation.
In the art of hydraulic fracturing subsurface earth formations from subterranean wells at depth, it is well known that the earth's compressive stresses at the region of fluid injection into the formation will typically result in the creation of a vertical two “winged” structure. This “winged” structure generally extends laterally from the well bore in opposite directions and in a plane generally normal to the minimum in situ horizontal compressive stress. This type of fracture is well known in the petroleum industry as that which occurs when a pressurized fracture fluid, usually a mixture of water and a gelling agent together with certain proppant material, is injected into the formation from a well bore which is either cased or uncased. Such fractures extend radially as well as vertically until the fracture encounters a zone or layer of earth material which is at a higher compressive stress or is significantly strong to inhibit further fracture propagation without increased injection pressure.
It is also well known in the prior art that the azimuth of the vertical hydraulic fracture is controlled by the stress regime with the azimuth of the vertical hydraulic fracture being perpendicular to the minimum horizontal stress direction. Attempts to initiate and propagate a vertical hydraulic fracture at a preferred azimuth orientation have not been successful, and it is widely believed that the azimuth of a vertical hydraulic fracture can only be varied by changes in the earth's stress regime. Such alteration of the earth's local stress regime has been observed in petroleum reservoirs subject to significant injection pressure and during the withdrawal of fluids resulting in local azimuth changes of vertical hydraulic fractures.
The method of controlling the azimuth of a vertical hydraulic fracture in formations of unconsolidated or weakly cemented soils and sediments by slotting the well bore or installing a pre-slotted or weakened casing at a predetermined azimuth has been disclosed. The method disclosed that a vertical hydraulic fracture can be propagated at a pre-determined azimuth in unconsolidated or weakly cemented sediments. See U.S. Pat. No. 6,216,783 to Hocking et al., and U.S. Pat. No. 6,443,227 to Hocking et al. The method disclosed that a vertical hydraulic fracture can be propagated at a pre-determined azimuth in unconsolidated or weakly cemented sediments. These methods in the prior art were not concerned with the creation of multiple orientated vertical hydraulic fractures at differing azimuths from a single well bore for the enhancement of petroleum fluid production from the formation.
Accordingly, there is a need for a method and apparatus for controlling the differing azimuth orientations of multiple vertical hydraulic fractures in a single well bore in formations of unconsolidated or weakly cemented sediments. Also, there is a need for a method and apparatus that hydraulically connects the installed vertical hydraulic fractures to the well bore without the need to perforate the casing.